Electrostatic assist printing has been used for sometime, particularly in the rotogravure field. As is well known, many of the inks used in gravure printing including electrostatic assist rotogravure printing, contain solvents which are very volatile, flammable, and capable of forming an explosive mixture with air which could be set off by a spark or fire. In electrostatic assist printing, a high voltage (say 500 volts or more) charge is transmitted from a generating power pack to a conductive material on, for example, an impression cylinder or roller. This charge on the impression roller helps draw the ink from the cells on the gravure cylinder onto the web or sheet which runs between the gravure cylinder and impression cylinders.
Various means, such as a simple contact made of copper sheet, paddle contact, single brush (see U.S. Pat. No. 4,697,514), or corona bar have been used to charge the conductive surface of the impression cylinder. As it is possible to have a spark occur with such contact mechanisms, heretofore attempts have been made to eliminate or minimize the problem by designing the power pack with safety circuitry so that as soon as a current surge is detected, as would occur were a spark eminent, the power pack is shut down to prevent the occurance of a spark. While such systems have worked well, they are not foolproof, and were the safety circuit to malfunction, a disastrous spark might occur.